Cross flow thermal oil recovery process



X daliiifmlfj Fwwlz April 27, 3%5 5.1. WiLLMAN CROSS FLOW THERMAL OIL RECOVERY PROCESS Filed Dec. 51, 1982 2 Sheets-Sheet 1 lnvenior Bertram T. Willmon By a Q Attorney 3,180.413 CROSS FLQW THERMAL OIL RECOVERY PROCESS Bertram T. Willman, Corpus Christi, Tern, assignor to Jersey Production Research Company, a corporation of Delaware Filed Dec. 31, 1962, Ser. No. 248,808 6 Claims. (Cl. 166-41) This is a continuation-in-part of application Serial No. 843,994, filed in the United States Patent Ofiice on Octoher 2, 1959, and now abandoned.

The present invention relates to methods for the recovcry of crude oil from subsurface reservoirs and is particularly concerned with oil recovery processes wherein heated fluids are injected into reservoirs in order to displace the oil contained therein.

Oil recovery methods utilizing steam, hot water and other heated fluids have been used in the past for stimulating production from reservoirs having low natural energy levels and for obtaining additional production from reservoirs whose natural energy has been expended during primary recovery operations. Such methods in general involve injection or" the steam, hot water or other fluid into the formation through one or more injection wells and the recovery of oil displaced by the injected fluid through one or more production wells drilled into the producing zone at points remote from the injection wells. Heat transferred to the oil in place brings about areduction in the viscosity of the oil and in some cases leads to distillation and cracking which further improve mobility of the oil. Efiicient displacement of the oil in place is therefore obtained.

Although processes of this type generally permit the recovery of more oil than can be obtained by low tempera ture techniques, such processes have drawbacks from an economic standpoint because of the high cost of supplying the large amounts of heat required. The heat energy supplied to an oil-bearing reservoir by the injection of steam, hot water or a similar heated fluid is not confined to the reservoir itself and instead is in part transmitted to the overlying and underlying strata adjacent the producing zone, where it is dissipated. The energy which does not escape from the reservoir in this manner is largely spent in raising the temperature of the rock making up the formation and the water present therein. Studies have shown that only about to about 25 percent of the total heat input to a reservoir is normally employed for elevating the temperature of oil and gas which are ultimately recovered. For these reasons, the heat requirements in such processes are generally high. Various methods for improving heat utilization and thus reducing the heat input requirements have been suggested from time to time but to date no satisfactory means for accomplishing this have been found.

The present invention provides an improved method for recovering oil from subsurface reservoirs by the injec tion-of'heatcd fluids which improves heat utilization and thus permits recovery at higher rates and in greater quan tities than might otherwise be possible. In accordance with the invention, it has been found that the disadvantages associated with steam injection, hot water injection, and similar processes can be partially avoided in reservoirs separated into vertically-spaced zones by injecting the steam, hot water or other fiuid in a manner such that countercurrent fiow in adjacent zones of the reservoir is obtained. Countercurrent flow of the injected high temperaturc fluid in adjacent zones of the reservoir results in an interchange of heat between the area surrounding the injection well in one zone and the area surrounding the production well in the adjacent zone. Thermal conduction through the strata separating the zones thus permits the utilization of heat energy which would otherwise $335,413 Patented Apr. 27, 1965 be lost without contributing to the oil recovery process. The result is a greater reduction in oil viscosity than could be obtained with the same total heat input if fluid were injected into the two zones in a concurrent flow pattern. The reduced viscosity permits increased injection rates and accelerates the flow of fluids through the formation from the injection wells to the production wells. Total oil recovery is increased because of the more uniform heating of the formation and the corresponding increase in erliciency of the injected fluid as an agent for displacing the oil contained therein.

The method of the invention may be carried out by employing separate injection and production wells peuetrating each zone or by utilizing multiple completion wells for both injection and production purposes. Packers may be placed in wells of the latter type at levels opposite the impervious strata in order to separate the wells into discrete zones. Production tubing and injection tubing may then be installed in each well at alternate levels between the packers in order to inject steam, hot water or a similar heated fluid into and recover all from alternate vertically-spaced zones. At least two different wells are used and the injection and production levels in each are selected so that countercurrent flow in adjacent oil producing zones is obtained. Conventional dual or multiple completion apparatus may be utilized to permit simultaneous injection and production in each well without intermixing of the oil and the injected steam or hot water.

The method of the invention is designed for use in reservoirs separated into a plurality of vertically-spaced oil-, bearing zones by impervious strata. The permissible thickness of the separating strata will depend largely upon the heat transfer characteristics of the material making up the strata and the period of time over which the contemplated recovery operation is to be carried out. The temperature at which the heated fluids are injected into the reservoir from the surface is also an important consideration. In general it has been found that the method is most effective in reservoirs wherein the separating strata are from about 1 to about feet thick, although in some cases countercurrent flow is advantageous in reservoirs having separating strata ranging up to about 250 feet in thickness. The relationship between the. thickness of the barrier between the adjacent zones. the temperature conditions in the zones, and the duration of the recovery operation will be explained in greater detail hereafter.

The process of the invention may be carried out with steam. hot water, hot combustion gases generated at the surface and other heated fluids but is particularly advantageous where steam is used because of the high temperatures obtained and the large quantities of heat transmitted to the reservoir. The process is particularly efi'ective for the recovery of relatively heavy oils, such as those having viscosities above about 50 centipoises under normal reservoir conditions of temperature and pressure.

The nature and objects of the invention can best be understood by referring to the following detailed description of a preferred embodiment of the process and to the accompanying drawing, in which:

FIGURE 1 is a schematic diagram of a vertical section through an oil bearing reservoir in which steam is utilized in accordance with the invention; and

FIGURE 2 is a graph illustrating the relationship between the thickness of the barrier separating the oil hearing zone in the reservoir, the duration of the oil recovery operation and the temperature rise obtained due to heat transfer through the barrier.

As can be seen from FIGURE 1 of the drawing, wells I and II penetrate through the overburden 10 and extend through oil-bearing sands l1 and 12, separated by shale or a similar oil-impervious barrier 13. Oil reservoirs often occur as thick gross sand bodies divided into 'discrete sections of oil bearing sand and intcrbedded shale or similar material. The shale can often be correlated from well to well in a given field. The reservoir depicted in FIGURE 1 is such a reservoir. Each of the wells shown in FIGURE 1 is provided Witt casing 14 which has been cemented in place in the upper part of the well, the cement being represented by reference numeral 15. The casing in each well has been perforated at points 16 within oil sands 11 and 12 by conventional methods. A string of tubing 17 extends downwardly within the casing through oil sand 11 and barrier 13 in each well. A packer 18, which may be of conventional design, is shown positioned between casing 14 and the tubing string 17 opposite im-' pervious barrier 13 in each of the wells. The packers prevent fluid in the annulus between the casing and tubing string above the barrier from flowing into the annular space within the casing below the barrier. A perforated screen or liner 20 is attached to the lower end of the tubing 17 below packer 18 in each well. The screen in each well is closed off at its lower end by shoe 21. Each well is provided with a casing head 22 and a Christmas tree 23. shown in simplified form, which permits separate handling of fluids in the tubing string and those in the annular space between the casing and tubing. It will be understood that the apparatus thus shown is representative of equipment which may be employed in carrying out the method-of the invention but that other forms of apparatus designed to provide multiple conduits within a wellbore may be utilized. In some cases it will be preferred to provide side-by-side conduits rather than the concentric conduitsshown in the drawing. Heat transfer between the fluids in the wellbore is somewhat higher when concentric conduits are'employed than it is if two separate tubing strings are used. Many recovery operations can be carried out without the use of easing opposite the producing zone and in high temperature operations it will frequently be preferable to dispense with casing in the lower part of the wells. Separate injection and production wells may be utilized in lieu of the dual completion wells shown if desired.

The steam utilized in the process depicted in FIGURE 1 of the drawing may be either saturated or supersaturated and may be injected into the reservoir at temperatures between about 300 F. and about l600 F. and at pressures between about 100 p.s.i. and about 4000 p.s.i. The upper pressure limit depends largely upon the formation breakdown pressure of the reservoir being treated. This in turn is governed by the depth of the reservoir, the characteristics of the reservoir rock, and the type of overburden. The lower pressure limit is determined by the'reservoir pressure, since the steam employed must obviously be injected at a pressure in excess of the reservoir pressure.

Injection pressures in the range between about 300 p.s.i.

and about 1500 p.s.i. are gcnerally prcferred. Saturated steam is usually more attractive for use in secondary re covery operations than is superheated steam, although the latter generally permits heat to be supplied to the reservoir at higher rates. Preferred temperatures when saturated steam is employed generally range between 450 and about 650 F.

The steam utilized in the process, at a pressure of about 1550 p.s.i. absolute and a temperature of about 600 F. for example, is injected from the surface into oil sands 11 and 12 in FIGURE 1 through wells I and II respectively. Steam passing downwardly through the annular space between casing 14 and tubing 17 above packer 18 in well I flows into sand section 11. Steam simultaneously injected into well It flows downwardly through the tubing string 17 into screen 20, passing through ports 18 in the casing outwardly into sand section 12. The upper sand section 11 surrounding well I and the lower sand section 12 surrounding well H are thus heated. Due to the temperature differences established in this manner, heat will be cona ducted upwardly through barrier 13 into the cold section of sand 1.! in the vicinity of well II and downwardly through barrier 13 into the cold section of sand 12 in. the

vicinity of well I. Simultaneously, oil is displaced from each of these sand sections by the injected steam.

The injection of steam and consequent displacement of oil into the reservoir as described above results in countercurrent flow of oil, steam and water in sand sections 11 and 12. After sutlicient steam has been introduced into each of the sand sections, the displaced oil begins to flow into the production zones of the two wells. Oil produced from sand section 11 passes through port 16 in casing 14 of well ii and is withdrawn to the surface through the annulus between the casing and tubing 17. Similarly, oil displaced from sand section 12 passes through port 16 in casing 14 of well I into stream 20 and is withdrawn through tubing 17. Pumps actuated by sucker rods from the surface or similar pumping equipment will normally be provided for lifting the oil to the surface. Suitable pumps and details concerning their operation are well known to those skilled in the art.

Production from the two wells shown in FIGURE 1 of the drawing is accelerated as a result of the countercurrent flow of hot fluids in adjacent oil bearing zones 11 and 12. Heat liberated by the steam injected into zone Ill through well I is in part utilized to heat the cold sand in zone 12 in advance of the steam injected therein. In like manner, heat liberated by the steam injected into zone 12 through well II serves in part to heat the cold portion of zone 11 before the steam injected through well I reaches it. This utilization of heat transmitted through the impervious barrier between the two zones reduces the amount of heat lost to formations above and below the reservoir and thus improves the economics of the oil recovery process. The reduction in the viscosity of the oil surrounding the production well in each zone permits the oil to liow out of the zone into the production well more readily than would be the case if the advancing steam had to force cold, relatively viscous oil through the formation. This makes possible the injection of steam at higher rates than could be obtained if the oil bearing zones were treated independeutly or if concurrent liow were used in the two zones. Because the reduced viscosity obtained permits the oil to how more readily without fingering," stratilication and aerial elticiency are improved and recovery is increased.

Similar benefits are obtained by utilizing hot water, a hot solvent or other heated tluid in place of steam for the displacement of oil from the two adjaccnt zones. The process described may be carried out with a plurality of multiple completed wells arranged in a suitable pattern over a large field and is applicable to reservoirs containing more than two discrete oil-bearing zones. The wells may be located in rive-spot, seven-spot or other conventional patterns.

As pointed out earlier, the heat transferred between the adjacent oil-bearing zones of the reservoir depends in part upon the thickness of the barrier separating the zones, the temperature rise ellcctcd within the zones due to the injection of steam, hot water or the like from the surface, and the period over which heat transfer through the barrier takes place. l-IGURE 2 of the drawing is a graph showing the relationship between these factors. The curves shown in FIGURE 2 are based upon countercurrent how in a reservoir containing two discrete producing zones separated by an intervening shale barrier. The curves indicate the temperature rise which will occur with a given thickness of shale over a given period due to heat transfer through the shale. The temperature rise in the zone to which heat is transmitted through the barrier is expressed as a percentage of the temperature rise occurring in the adjacent zone due to the injection of steam. hot water or a similar heated fluid. it can be seen from the curves that steam injected into one zone of the reservoir at a temperature of 500 F. above the initial formation temperature will produce a ll) percent, or 50 rise in temperature in the adjacent zone well if the thickness of the barrier is 58 feet and the heat transfer continues for a period of three years. The temperature rise which will occur at points between the wells will be less than that at points near the wells because the period during which heat transfer through the barrier occurs will be shorter. Because the hot zones in the two adjacent formations approach each other during the recovery process, however, heat transfer takes place over the entire recovery pattern. In the critical area surrounding the production wells where the stream lines of the producing fluids converge, heating is a maximum.

Assuming that the temperature increase due to the injection of steam, hot water or the like is 500 R, FIG- URE 2 of the drawing shows that a temperature rise of 50 F. would be obtained in an adjacent zone 105 feet away during a year heat transfer period. In like manner, it a 50 rise in temperature due to heat transmitted through the shale barrier is necessary in order to obtain a significant increase in oil viscosity, if the heated fluid injected into the reservoir from the surface causes a temperature rise of 200 F., and if a year injection period is contemplated due to the size of the reservoir being produced, it can be seen from FIG- URE 2 that the necessary percent increase in temperature due to heat transfer through the barrier will not be obtained if the two zones of the reservoir are separated by more than about 103 feet of shale. If, on the other hand, the temperature rise due to injection of the steam is 1,000 E. sufiicient heat to produce a 50 F. increase in temperature in the adjacent bed during a 20 year period will be transmitted through a shale barrier 175 feet thick. The temperature increase which can be expected in a particular reservoir due to heat transfer through a shale barrier can thus be readily determined. The curves shown in FXGURE Z of the drawing are based upon a reservoir separated into discrete zones by an intervening layer of oilimpervious shale. Similar curves can be prepared for reservoirs in which the oilbearing zones are separated by impermeable materials having greater or lesser heat transfer coeficients than shale. Dilliercnces in the heat transfer coetficient produce changes in the amount of heat transmitted under a particular set of conditions but significant temperature increases are seldom obtained through barriers more than about 250 feet thick. The process of the invention is most effective when applied to reservoirs containing barriers less than about 150 feet in thickness.

What is claimed is: 1. A process for the recovery of oil from a subsurface reservoir containing two vertically-spaced oil-bearing zones separated by a substantially oil-impervious barrier less than about 250 feet thick which comprises:

injecting a fluid heated at the earths surface to a temperature in excess of the normal temperatures within said oil-bearing zones into a first of said zones through a well completed into said first zone;

injecting a fluid heatedat the earths surface to a temperature in excess of the normal temperatures within said oil-bearing zones into a second of said zones through a well completed into said second zone at a point remote from the injection point in said first zone;

recovering oil displaced by said fluid injected into said first zone through a well completed into said first zone in the vicinity of the injection point in said second zone;

and simultaneously recovering oil displaced by said fluid injected into said second zone through a well completed into said second zone in the vicinity of the injection point in said first zone.

2. A process as defined by claim 1 wherein said fluid injected into said first zone and said fluid injected into said second zone are steam.

3. A process as defined in claim 1 wherein said fluid injected into said first zone and said fluid injected into said second zone are hot water.

4. A process for the recovery of oil from a subsurface reservoir containing two vertically-spaced oil-bearing zones separated by a substantially oil-impervious barrier less than about 250 feet thick which comprises:

injecting a heated aqueous fluid into a first of said oilbearing zones through a first well penetrating said zones; injecting a heated aqueous fiuid into a second of said oil-bearing zones through a second well penetrating said zones at a point remote from said first well;

recovering oil displaced by said heated fluid injected into said first zone through said second well;

and simultaneously recovering oil displaced by said heated fluid injected into said second zone through said first well.

5. A process for the recovery of oil from a subsurface reservoir containing two vertically-spaced oil-bearing zones separated by a substantially oil-impervious barrier less than about 250 feet thick which comprises:

injecting steam. from the surface into a first of said oilbearing zones through a first well penetrating said zones;

injecting steam from the surface into a second of said oil-bearing zones through a second well penetrating said zones at a point remote from said first well; recovering through said second well oil displaced from said first zone by steam injected at said first well; and simultaneously recovering through said first well oil displaced from said second zone by steam injected at said second well.

6. A process for the recovery of oil from a subsurface reservoir containing two vertically-spaced oil-bearing zones separated by a substantially oil-impervious barrier less than about 250 feet thick which comprises:

injecting hot water from the surface into a first of said oil-bearing zones through a first well penetrating said zones;

injecting hot water from the surface into a secondof said oil-bearing zones through a second well penetrating said zones at a point remote from said first well;

recovering through said second well oil displaced from said first zone by hot water injected at said first well;

and recovering through said first well oil displaced from said second zone by hot water injected at said second well.

References Cited by the Examiner UNITED STATES PATENTS 2,217,749 ill/40 Hewitt 166-10 X 2,734,579 2/56 iiikins l66-ll 3,042,114 7/62 Willman 16640 X 3,054,448 9 62 Dew et al. 166-41 BENJAMIN HERSH. Primary Examiner. CHARLES E. OCONNELL, Examiner. 

1. A PROCESS FOR THE RECOVERY OF OIL FROM A SUBSURFACE RESERVOIR CONTAINING TWO VERTICALLY-SPACED OIL-BEARING ZONES SEPARATED BY A SUBSTANTIALLY OIL-IMPERVIOUS BARRIER LESS THAN ABOUT 250 FEET THICK WHICH COMPRISES: INJECTING A FLUID HEATED AT THE EARTH''S SURFACE TO A TEMPERATURE IN EXCESS OF THE NORMAL TEMPERATURES WITHIN SAID OIL-BEARING ZONES INTO A FIRST OF SAID ZONES THROUGH A WELL COMPLETED INTO SAID FIRST ZONE; 